A gas lift system and a gas lift method

ABSTRACT

The present invention relates to a gas lift system ( 1 ) arranged in a well ( 65 ) having a top ( 10 ) and comprising a well fluid ( 11 ), the gas lift system comprising a casing ( 61 ) arranged in the borehole defining a surrounding annulus ( 62 ), an opening ( 86 ) in the casing, and a pumping unit ( 12 ) for pumping gas ( 8 ) into the annulus, wherein the gas lift system further comprises a downhole tool ( 14 ) having a first end ( 15 ) nearest the top of the well and a second end ( 16 ), the down-hole tool comprising a pump section ( 2 ) comprising a housing ( 20 ) comprising a chamber ( 201 ) having an axial extension ( 17 ), a plunger ( 23 ) sliding in the chamber and dividing the chamber into a first compartment ( 203 ) and a second compartment ( 202 ), a plunger rod ( 26 ) connected with the plunger, a chamber inlet channel ( 27 ) and a chamber outlet channel ( 21 ), the inlet channel and the outlet channel being in fluid communication with the second compartment, a sealing element ( 29 ) for isolating a first part ( 66 ) of the casing from a second part ( 67 ) of the casing, the first part and the inlet channel being in fluid communication with the opening, a one-way valve ( 24 ) arranged in the inlet channel to allow fluid to flow into the chamber, and a one-way valve ( 22 ) arranged in the outlet channel to allow fluid to flow out of the chamber. The invention also relates to a gas lift method.

FIELD OF THE INVENTION

The present invention relates to a gas lift system arranged in a wellhaving a top and comprising a well fluid, the gas lift system comprisinga casing arranged in the borehole defining a surrounding annulus, anopening in the casing, and a pumping unit for pumping gas into theannulus. The invention also relates to a gas lift method.

BACKGROUND ART

Gas lifting of the fluid column in a well to overcome a hydro-staticpressure and drive hydrocarbons to surface is typically performed bymeans of side pockets arranged as part of the production casing. Theannulus created between the production casing and the intermediatecasing is pressurised with gas which, at certain pressures, is allowedto flow in through a certain side pocket gas lift valve. Pressurised gasis able to flow down the annulus until a certain point, at which pointthe last side pocket is normally arranged. At present, it is impossibleto pressurise the gas to flow past this point.

SUMMARY OF THE INVENTION

It is an object of the present invention to wholly or partly overcomethe above disadvantages and drawbacks of the prior art. Morespecifically, it is an object to provide an improved gas lift systemwhich is able to provide gas lift further down the well beyond the pointwhich can be reached by known systems having side pocket gas liftvalves.

The above objects, together with numerous other objects, advantages andfeatures, which will become evident from the below description, areaccomplished by a solution in accordance with the present invention by agas lift system arranged in a well having a top and comprising a wellfluid, the gas lift system comprising:

-   -   a casing arranged in the borehole defining a surrounding        annulus,    -   an opening in the casing, and    -   a pumping unit for pumping gas into the annulus,        wherein the gas lift system further comprises a downhole tool        having a first end nearest the top of the well and a second end,        the downhole tool comprising a pump section comprising:    -   a housing comprising a chamber having an axial extension,    -   a plunger sliding in the chamber and dividing the chamber into a        first compartment and a second compartment,    -   a plunger rod connected with the plunger,    -   a chamber inlet channel and a chamber outlet channel, the inlet        channel and the outlet channel being in fluid communication with        the second compartment,    -   a sealing element for isolating a first part of the casing from        a second part of the casing, the first part and the inlet        channel being in fluid communication with the opening,    -   a one-way valve arranged in the inlet channel to allow fluid to        flow into the chamber, and    -   a one-way valve arranged in the outlet channel to allow fluid to        flow out of the chamber.

By having a suction tool within the casing, the gas can enter furtherdown the well, and the system is thus capable of providing gas lift onmore fluid than in the known systems. The present system is thus capableof providing gas lift of wells which cannot be gas lifted by the knownsystems.

Furthermore, the downhole system may also be submerged into deep waterwells to decrease the pressure in these wells so that hydrate does notform in the well head or blowout preventer. Hydrate is formed whenhaving low temperature and high pressure and a certain amount of waterin the well fluid.

Also, at least part of the inlet channel may extend radially from anouter surface of the housing towards the chamber.

Moreover, the plunger may comprise a protrusion protruding from a firstplunger face of the plunger facing the inlet channel and/or the outletchannel.

The protrusion may be arranged, extending from an end face of thechamber nearest the second end of the tool.

In addition, the sealing element may be an annular seal for sealingaround the opening.

Furthermore, a one-way valve may be arranged in the opening to allowfluid to flow into the casing.

The sealing element may also circumferent the housing and may be adaptedfor sealing against the casing.

The gas lift system according to the invention may further comprise asecond sealing element circumferenting the housing and adapted forsealing against the casing, thereby isolating a part of the first partof the casing opposite the opening.

Moreover, an inlet of the inlet channel may be arranged substantiallyopposite the opening along the axial extension.

Additionally, the inlet of the inlet channel may be arrangedsubstantially opposite the opening along a circumference of the housing.

Further, an outlet of the outlet channel may be arranged closer to thefirst end of the tool than the inlet channel.

In addition, an outlet of the outlet channel may be arranged closer tothe first end of the tool than the inlet.

The gas lift system according to any of the invention may furthercomprise a linear actuator for providing a reciprocating movement of theplunger.

The linear actuator described above may comprise:

-   -   a tubular stroker cylinder comprising one or more piston        housings,    -   one or more piston elements slidably disposed in the piston        housing to divide the piston housing into a first chamber and a        second chamber,    -   a stroker shaft operably connected to the piston element for        connecting with a plunger rod to provide reciprocation of the        plunger,    -   a pump for alternately supplying hydraulic fluid under pressure        to the first chamber and the second chamber of the piston        housing to reciprocate the piston element in the piston housing,        and    -   an electrical motor for driving the pump.

Furthermore, the sealing element may be a chevron seal.

Also, the first end of the tool may be connected with a wireline.

Moreover, the opening may comprise two successive one-way valves.

The casing may also comprise a plurality of openings spaced apart alongthe casing.

Additionally, the sealing element may be inflatable or expandable.

The tool may comprise a driving unit for propelling the tool forward inthe well.

In addition, the tool may comprise a tool section comprising a drillingbit for drilling or milling an opening in the casing.

Further, the casing may comprise at least one side pocket, and the sidepocket may be arranged closer to the top than the opening.

The casing may have a substantially vertical part and a substantiallyhorizontal part and a toe part connecting the vertical part and thehorizontal part, and the opening may be arranged in the toe part or thehorizontal part.

A bore may extend from the second end of the tool to an equalising valvearranged in the outlet channel to connect the bore and part of theoutlet channel.

The present invention also relates to a gas lift method comprising thefollowing steps:

-   -   inserting the gas lift system according to any of the preceding        claims into the borehole,    -   arranging the inlet channel opposite the opening,    -   isolating the first part of the casing from the second part of        the casing by means of a sealing element, so that the first part        is in fluid communication with the opening and the inlet channel        is in fluid communication with the opening,    -   activating the pump unit to pump gas into the annulus,    -   moving the plunger rod in a reciprocating movement, and    -   sucking fluid from the first part of the casing, so that the gas        in the fluid in the annulus is sucked towards the opening and        into the first part and further into the first compartment and        out through the outlet channel.

The gas lift method described above may further comprise the step ofisolating a part of the first part of the casing opposite the opening tocreate a suction area between the casing and the tool opposite theopening.

The method may further comprise the step of making the opening in thecasing.

Further, the gas lift method may further comprise the step of drillingan opening in the casing.

Also, the method may comprise the step of releasing the tool from thecasing.

Moreover, the method may comprise the step of making a second opening inthe casing.

Additionally, the method may comprise the step of moving the toolfurther away from the top of the well to be opposite a second openingand engaging the casing by means of the sealing element.

Finally, the method may comprise the step of sucking fluid in throughthe second opening.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its many advantages will be described in more detailbelow with reference to the accompanying schematic drawings, which forthe purpose of illustration show some non-limiting embodiments and inwhich

FIG. 1 shows a perspective view of a gas lift system in a well,

FIG. 2 shows a cross-sectional view of part of a tool,

FIG. 3 shows a cross-sectional view of part of another tool,

FIG. 4 shows a cross-sectional view of part of a linear actuator,

FIG. 5 shows a cross-sectional view of part of another linear actuator,

FIG. 6 shows a partly cross-sectional view of another tool,

FIG. 7 shows a perspective view of another gas lift system in a well,

FIG. 8 shows a partly cross-sectional view of part of the gas liftsystem, and

FIG. 9 shows another gas lift system arranged in a well.

All the figures are highly schematic and not necessarily to scale, andthey show only those parts which are necessary in order to elucidate theinvention, other parts being omitted or merely suggested.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a gas lift system 1 arranged in a well 65 for providing gaslift of well fluid to overcome a hydro-static pressure and drivehydrocarbon-containing fluid, such as oil, to surface at a top 10 of thewell. The gas lift system comprises a casing 61 arranged in the welldefining a surrounding annulus 62 and further comprises a pumping unit12 arranged at the top, e.g. near the well head or blowout preventer,for pumping gas 8 into the annulus. The gas lift system furthercomprises a downhole tool 14 having a first end 15 nearest the top ofthe well and a second end 16 arranged near an opening 86 in the casing61. The downhole tool comprises a housing 20 in which a pump section 2is arranged. The pump section comprises a chamber 201 having an axialextension 17. The tool further comprises a plunger 23 sliding in thechamber and dividing the chamber into a first compartment 203 and asecond compartment 202. The plunger is connected with a plunger rod 26and is moved in a reciprocating motion for suction of fluid in throughthe opening 86. The chamber has a chamber inlet channel 27 and a chamberoutlet channel 21, the inlet channel and the outlet channel both beingin fluid communication with the second compartment. A sealing element 29is arranged to isolate a first part 66 of the casing from a second part67 of the casing, the first part and the inlet channel being in fluidcommunication with the opening 86 in the casing for letting gas from theannulus into the fluid in the casing.

When using the gas lift system shown in FIG. 1, in which a suction tool14 in the form of the downhole tool having the pump section is arrangedwithin the casing, the gas can enter further down the well than in knownsystems, simply by using gas lift through side pockets, and the systemis thus capable of providing gas lift further down the well than in theknown systems, and more fluid may thus be lifted so that the well can beself-producing again. The present system is thus able to gas lift wellswhich cannot be gas lifted by the known systems in order to produceagain.

Furthermore, the downhole system can also be submerged into deep waterwells to decrease the pressure in these wells so that hydrate does notform in the well head or blowout preventer. Hydrate is formed whenhaving low temperature and high pressure and a certain amount of waterin the well fluid.

As shown in FIG. 2, a one-way valve 24 is arranged in the inlet channel27 to allow fluid to flow into the chamber, and a one-way valve 22 isarranged in the outlet channel to allow fluid to flow out of thechamber. Thus, by moving the plunger in a reciprocating movement, fluidis sucked in through the inlet channel and ejected through the outletchannel. In this way, gas-containing fluid in the annulus is sucked intothe first part of the casing and provides gas lift of the well fluid inthe casing to overcome the hydro-static pressure and drive thehydrocarbons to surface.

Due to the fact that in known pumping systems, gas can only be pumped toa certain point down the annulus, gas lift can only be provided down tothis point, which in some wells results in the well not being capable ofproducing again. By inserting a tool opposite the opening arrangedfurther down the well than the certain point, the tool aids thegas-containing fluid in the annulus further down beyond the certainpoint by sucking fluid into the chamber just opposite the opening. It isthus possible to provide gas lift further beyond the certain point,which is needed when the known gas lift systems are not able to overcomethe hydro-static pressure and drive the hydrocarbons to surface throughside pocket gas lift mandrels. Furthermore, by inserting the tool intothe casing opposite the opening, side pocket gas lift mandrels are nolonger needed in order to provide gas lift. When having side pocketsinstalled in the casing, the platform at the sea bed or surface has tobe made sufficiently larger, because the cross-sectional diameter of thecasing at the side pockets is substantially increased. The making ofsuch platforms with side pockets is substantially more costly thancasings without the side pockets. Therefore, the present gas lift systemalso results in wells no longer having to be made with side pocketsinitially, and thus the costs involved in completing a well aresubstantially reduced.

As shown in FIG. 2, the inlet channel extends radially from an outersurface 36 of the housing towards the chamber. The inlet channel mayhave a radial extending part and an axial extending part, so that fluidenters radially and is then let axially into an outlet of the inletchannel arranged in an end face of the chamber. The plunger comprises aring-shaped protrusion 232 protruding from a first plunger face 231 ofthe plunger facing the inlet channel and/or the outlet channel. Theprotrusion may have any kind of cross-sectional shape and may also bearranged extending from an end face 204 of the chamber nearest thesecond end of the tool. The protrusion ensures a certain minimumdistance between the plunger face and the end face of the pump chamber,thereby preventing the plunger from blocking the outlet provided in thewall of the pump housing. Furthermore, a strainer element 88 may beprovided for filtrating well fluid before such well fluid enters thechamber or the inlet. For illustrative purposes, the strainer element 88is shown to be arranged at an inlet 52 of the inlet channel 27.

The pump section shown in FIGS. 3 and 7 further comprises a secondsealing element 35 circumferenting the housing and adapted for sealingagainst the casing, thereby isolating a part of the first part of thecasing opposite the opening. Thus, the sealing elements 29, 35 aredisposed around the housing on opposite sides of the inlet 52. Thesealing elements are configured to provide a seal between the tool andthe casing, thereby sealing off a section 87 of the casing as shown inFIGS. 6 and 7. Thus, the gas-containing fluid from the annulus is drawnin through the chamber in the tool and out through the outlet 53 closerto the first end 15 of the tool as shown in FIG. 1.

The tool 14 further comprises a linear actuator 40 arranged inassociation with the housing 20, as shown in FIG. 1. As shown in FIG. 4,the linear actuator 40 comprises a tubular stroker cylinder 4 defining apiston housing 47 and a piston element 46 slidably disposed in thepiston housing to divide the piston housing into a first chamber 41 anda second chamber 42. A stroker shaft 45 extending from the pistonelement is operably connected with the plunger rod of the pump sectionto provide reciprocation of the plunger in the chamber. The linearactuator further comprises a pump 5 (shown in FIG. 1) for alternatelysupplying hydraulic fluid under pressure to the first chamber 41 and thesecond chamber 42 of the tubular stroker cylinder, and an electricalmotor 6 (shown in FIG. 1) is provided for driving the pump. When fluidis alternately supplied to the first chamber 41 and a second chamber 42,the piston element is reciprocated in the tubular stroker cylinder,thereby creating a linear motion. The linear motion is transferred viathe stroker shaft to the plunger rod 26 (shown in FIGS. 2 and 3),thereby reciprocating the plunger in the chamber, causing a pumping orsucking effect to be created.

Referring to the embodiment of a pump section 2 shown in FIG. 2, whenthe plunger is reciprocated, well fluid with gas is drawn into the pumpchamber 201 through the inlet channel 27 in the lower part of thechamber, forced through the one-way valve 24, and expelled through theoutlet channel 21, also in the lower part of the chamber. Morespecifically, during an upstroke motion, the plunger moves away from theinlet channel 27 and the one-way valve 24, resulting in well fluidopposite the opening being sucked in through the inlet channel 27, pastthe open one-way valve 24 and into the second compartment 202 of thechamber. The one-way valve in the inlet channel 27 is a check-valve onlyallowing fluid to flow into the chamber, and the one-way valve in theoutlet channel is closed, since fluid is only allowed to flow out of theone-way valve in the outlet channel 21. Thus, as the plunger 23 reachesan upper extreme position, the second compartment 202 of the chamber hasbeen flooded and the first compartment almost emptied. Fluid in thefirst compartment 203 is forced out through an aperture 18 in a chamberwall 19 and let out into the casing. A subsequent down-stroke motion ofthe plunger, wherein the plunger moves towards the inlet channel 27 andthe one-way valve 24, forces the fluid through the outlet one-way valve24 and thus out into the casing as the inlet one-way valve is closed. Inthe shown embodiment, the first valve and the second valve are embodiedas check-valves of the ball-type and comprise a displaceable valve ball221, 241 cooperating with a valve seat 222, 242 to control the flowdirection. However, the skilled person would know that many other typesof valves may be envisaged providing similar functionality. Further, thedesign of the pump section is based on the principles of widely used rodpumps, and other designs of the pump section may thus be envisaged bythe skilled person without departing from the invention.

Details about the design of the linear actuator are shown in FIGS. 4 and5 showing different embodiments of a linear actuator. In bothembodiments, the stroker shaft 45 extends through the tubular strokercylinder 4 sectioned into one or more piston housings 47 by partitions48. The partitions comprise a sealing means 49 b, such as an O-ring, inorder to provide a sealing connection between the partitions and thestroker shaft 45. In each of the piston housings 47, a piston element 46is provided around the stroker shaft 45, so that the stroker shaft 45may run back and forth within the tubular stroker cylinder 4 to providethe linear motion. Each of the piston elements 46 divides each of theone or more piston housings into a first chamber 41 and a second chamber42, and the piston elements are provided with sealing means 49a in orderto provide a sealing connection between the inside of the piston housing47 and the outside of the piston element 46. As shown in FIG. 4, fluidis alternately supplied to the first chamber 41 and the second chamber42 via the respective fluid channels 43, 44. In the embodiment shown inFIG. 5, only the fluid channels in fluid communication with the firstpiston housing are shown. However, the other piston housings areprovided with a similar arrangement of fluid channels. To provide thelinear motion of the linear actuator, the pump of the linear actuatorpumps fluid into the first chamber by sucking a corresponding amount offluid from the second chamber 42, and vice versa. When the first chamber41 is substantially filled, the pump shifts its pumping direction andpumps fluid from the first chamber 41 into the second chamber 42.Consequently, the piston element 46 is forced in the opposite direction.And hence the stroker shaft 45 is forced back and forth, therebyproviding the linear motion. As can be seen in FIG. 4, the first chamber41 is provided with a fluid channel 43 at one end of the piston housing47, and the second chamber 42 is provided with a fluid channel 44 at theopposite end of the piston housing 47. In this way, fluid can be suckedor pumped into each chamber until the piston element 46 almost abuts thepartitions 48. The linear actuator is thus a closed system, meaning thatthe same fluid is recirculated by being pumped back and forth in thepiston housing 47 in order to move the one or more piston elements 46back and forth.

In another embodiment, the linear actuator may comprise an electriclinear motor 51 driving the stroker shaft as shown in FIG. 7.Furthermore, a one-way valve 25 is arranged in the opening to allowfluid to flow into the casing but prevent the fluid from flowing fromthe casing into the annulus. The opening 86 may also be provided withtwo successive one-way valves to provide a safer solution.

In order to increase the amount of gas sucked into the casing, the inlet52 of the inlet channel is arranged substantially opposite the openingalong the axial extension as shown in FIG. 7. Thus, the inlet of theinlet channel is arranged substantially opposite the opening, also alonga circumference of the housing. The tool may comprise a plurality ofinlets in order to ensure that the orientation of the tool is notimportant.

As shown in FIG. 1, the outlet channel has an outlet 53, which isarranged closer to the first end 15 of the tool than the inlet channel27. In FIG. 1, the outlet 53 is arranged substantially above the chamberin the wall of the hydraulic cylinder, and in FIG. 7, the outlet isarranged in a connector 59 in the first end 15 of the tool.

In FIG. 8, the sealing element is an annular seal for sealing around theopening, and the sealing element has the shape of a cup seal surroundingthe opening 86.

The sealing elements in FIGS. 2 and 3 are chevron seals, and the sealingelements in FIGS. 1 and 7 are inflatable or expandable seals which aretypically made of elastomers or metal.

As can be seen in FIGS. 1 and 7, the tool of the gas lift system is awireline tool where the first end of the tool is connected with awireline 60.

The casing comprises a plurality of openings 86 a, 86 b, 86 c spacedapart along the casing as shown in FIG. 9. First the tool is arrangedopposite the first opening 86 a so that the sealing element is inflatedjust below the opening. After some time, when the gas-containing fluidhas entered through the first opening for some time, the tool and thesealing element are deflated, and the tool is lowered further down thewell to be opposite the second opening 86 b at which the sealing elementis inflated just below the second opening, and gas-containing fluid issucked in through the second opening. After some time, the tool islowered further down the well and inflates the sealing element, so thatthe inlet of the tool is arranged opposite the third opening 86 c asshown in FIG. 9. The tool may comprise a bit for making the openings.

As can be seen in FIG. 7, the tool comprises a driving unit 9 forpropelling the tool forward in the well. The driving unit is driven by apump 5 a which is driven by the motor 6.

In FIG. 9, the casing of the gas lift system comprises three sidepockets 54, and the side pockets are arranged closer to the top than theopening. The point 90 at which the gas can be introduced by means of theside pockets are at the bottom side pocket. The casing has asubstantially vertical part 91 and a substantially horizontal part 92and a toe part 93 connecting the vertical part and the horizontal part,and the opening is arranged in the toe part or the horizontal part.

In FIG. 3, a bore extends from the second end 16 of the tool to anequalising valve 37 arranged in the outlet channel 21 to connect thebore and part of the outlet channel. The equalisation valve is adaptedto equalise a differential pressure created across the sealing element29 when the sealing element 29 is set in the well, and gas-containingwell fluid is pumped into the first part of the casing above the sealingelement 29. To monitor and measure the differential pressure across thesealing element 29, the sealing element 29 or other parts of the toolmay comprise a system for measuring the differential pressure, such asone or more sensors 39 for measuring the pressure in the lower and theupper sections of the well. The differential pressure may, however, alsobe determined based on other principles known to the skilled person,inter alia based on the force required to drive the plunger in the pumpsection. When the gas lift system has lifted the fluid for some time,the well may to some extent be self-producing, so that the pressure inthe second casing part is higher than in the first casing part, and thefluid from the second part is thus let through the equalisation valveand out through the outlet, which helps overcome a hydro-static pressureof the fluid above the opening.

In FIG. 6, the no-go shoulder 95 provided in the tool interacts with arecess 96 in the casing 61 to fixate the position of the tool and ensurethat the pump section 2 is positioned in the correct position. The partof the casing comprising the recess 96 may be a landing nipple known tothe skilled person. As shown in FIG. 6, the sealing elements 29 arepositioned on opposite sides of an opening 86. Each sealing element 29hereby provides a seal in the annulus between the pumping assembly andthe casing in order to seal off a section 87 of the casing.

Further, as described above, the design of the pump section allows wellfluid to flow from the inlet 52 towards the outlet 53 regardless of theposition of the plunger 23. The tool 14 may thus be arranged in the welldownhole permanently or for longer periods of time, operating based onthe actual demand for boosting the flow in the well. If, for somereason, the flow in the well suddenly drops, the tool may be activatedto boost the flow until the well is once again able to run by itself.The tool may be activated either automatically based on a measuredpressure in the well or by a signal received from an operator throughthe wireline. The measured differential pressure across the set sealingelement may thus be used to control the operation of the pumping actionof the tool by continuously activating and deactivating the pumpingaction to boost the flow in the well by providing gas lift.

The tool 14 of the gas lift system 1 is thus lowered into the casing andarranged so that the inlet channel 27 is opposite the opening, then thefirst part 66 of the casing is isolated from the second part 67 of thecasing by means of a sealing element 29, so that the first part and theinlet channel are in fluid communication with the opening. Subsequently,the pump unit is activated to pump gas into the annulus, and the plungerrod is moved in a reciprocating movement in order to suck fluid from thefirst part of the casing, so that the gas in the fluid in the annulus issucked towards the opening and into the first part and further into thefirst compartment and out through the outlet channel.

When having two sealing elements 29, 35, a part of the first part of thecasing opposite the opening can be isolated to create a suction areabetween the casing and the tool opposite the opening. The tool mayfurther comprise means for making the opening in the casing downhole.

In the event that the gas lift provided through the first opening is notsufficient for well fluid to start flowing by the pressure present inthe reservoir, the tool releases itself from the casing by deflating orunexpanding the sealing elements and subsequently moves further down thewell and makes a second opening in the casing and initiates a suckingaction opposite the second opening and provides gas lift further downthe well from the first opening. Should the gas lift provided throughthe second opening not be sufficient, the process of deactivating thesealing elements and making an opening further down the well isrepeated. Thus, the tool may comprise a tool section 94 comprising adrilling bit 97 for providing such opening in the casing as shown inFIG. 9.

The linear actuator may be a stroking tool or stroker tool is a toolproviding an axial force. The stroking tool comprises an electricalmotor for driving a pump. The pump pumps fluid into a piston housing tomove a piston acting therein. The piston is arranged on the strokershaft. The pump may pump fluid into the piston housing on one side andsimultaneously suck fluid out on the other side of the piston.

By fluid or well fluid is meant any kind of fluid that may be present inoil or gas wells downhole, such as natural gas, oil, oil mud, crude oil,water, etc. By gas is meant any kind of gas composition present in awell, completion, or open hole, and by oil is meant any kind of oilcomposition, such as crude oil, an oil- containing fluid, etc. Gas, oil,and water fluids may thus all comprise other elements or substances thangas, oil, and/or water, respectively.

By a casing is meant any kind of pipe, tubing, tubular, liner, stringetc. used downhole in relation to oil or natural gas production.

In the event that the tool is not submergible all the way into thecasing, a downhole tractor can be used to push the tool all the way intoposition in the well. The downhole tractor may have projectable armshaving wheels, wherein the wheels contact the inner surface of thecasing for propelling the tractor and the tool forward in the casing. Adownhole tractor is any kind of driving tool capable of pushing orpulling tools in a well downhole, such as a Well Tractor®.

Although the invention has been described in the above in connectionwith preferred embodiments of the invention, it will be evident for aperson skilled in the art that several modifications are conceivablewithout departing from the invention as defined by the following claims.

1. A gas lift system arranged in a well having a top and comprising awell fluid, the gas lift system comprising: a casing arranged in theborehole defining a surrounding annulus, an opening in the casing, and apumping unit for pumping gas into the annulus, wherein the gas liftsystem further comprises a downhole tool having a first end nearest thetop of the well and a second end, the downhole tool comprising a pumpsection comprising: a housing comprising a chamber having an axialextension, a plunger sliding in the chamber and dividing the chamberinto a first compartment and a second compartment, a plunger rodconnected with the plunger, a chamber inlet channel and a chamber outletchannel, the inlet channel and the outlet channel being in fluidcommunication with the second compartment, a sealing element forisolating a first part of the casing from a second part of the casing,the first part and the inlet channel being in fluid communication withthe opening, a one-way valve arranged in the inlet channel to allowfluid to flow into the chamber, and a one-way valve arranged in theoutlet channel to allow fluid to flow out of the chamber.
 2. A gas liftsystem according to claim 1, wherein at least part of the inlet channelextends radially from an outer surface of the housing towards thechamber.
 3. A gas lift system according to claim 1, wherein the plungercomprises a protrusion protruding from a first plunger face of theplunger facing the inlet channel and/or the outlet channel.
 4. A gaslift system according to claim 1, wherein the sealing element is anannular seal for sealing around the opening.
 5. A gas lift systemaccording to claim 1, wherein a one-way valve is arranged in the openingto allow fluid to flow into the casing.
 6. A gas lift system accordingto claim 1, wherein the sealing element circumferents the housing and isadapted for sealing against the casing.
 7. A gas lift system accordingto claim 1, further comprising a second sealing element circumferentingthe housing and adapted for sealing against the casing, therebyisolating a part of the first part of the casing opposite the opening.8. A gas lift system according to any claim 1, wherein an inlet of theinlet channel is arranged substantially opposite the opening along theaxial extension.
 9. A gas lift system according to claim 1, wherein anoutlet of the outlet channel is arranged closer to the first end of thetool than the inlet channel.
 10. A gas lift system according to claim 1,further comprising a linear actuator for providing a reciprocatingmovement of the plunger.
 11. A gas lift system according to claim 1,wherein the tool comprises a driving unit for propelling the toolforward in the well.
 12. A gas lift system according to claim 1, whereinthe tool comprises a tool section comprising a drilling bit for drillingor milling an opening in the casing.
 13. A gas lift system according toclaim 1, wherein the casing comprises at least one side pocket, and theside pocket is arranged closer to the top than the opening.
 14. A gaslift method comprising the following steps: inserting the gas liftsystem according to claim 1 into the borehole, arranging the inletchannel opposite the opening, isolating the first part of the casingfrom the second part of the casing by means of the sealing element, sothat the first part is in fluid communication with the opening and theinlet channel is in fluid communication with the opening, activating thepump unit to pump gas into the annulus, moving the plunger rod in areciprocating movement, and sucking fluid from the first part of thecasing, so that the gas in the fluid in the annulus is sucked towardsthe opening and into the first part and further into the firstcompartment and out through the outlet channel.
 15. A gas lift methodaccording to claim 14, further comprising the step of isolating a partof the first part of the casing opposite the opening to create a suctionarea between the casing and the tool opposite the opening.
 16. A gaslift method according to claim 14, further comprising the step ofdrilling an opening in the casing.